1. Field of the Invention
The present invention relates to a wavelength converter with a fluorescent substance having semiconductor fine particles dispersed in a matrix, a lighting system in which the light emitted from a light emitting diode is subjected to wavelength conversion in a wavelength converter and taken out to the exterior, and a lighting system assembly provided with a plurality of the lighting systems. In particular, the present invention relates to a wavelength converter for lighting, and a lighting system which are suitably used in a backlight power source for an electronic display, or a fluorescent lamp, and the like.
2. Description of Related Art
A light emitting diode formed of a semiconductor material (hereinafter referred to as an “LED chip” in some cases) is small, has high power efficiency, and clearly develops colors. The LED chip has excellent characteristics of a long product life, high resistance to repetition of on/off lighting, and low power consumption. It is therefore anticipated that the LED chip will be applied to light sources for lighting such as a backlight light source of a liquid crystal or the like, and fluorescent lamps.
In the application of the LED chip to a light emitting unit, there has already been manufactured a light emitting unit emitting a color different from that of the light of the LED, in which part of the light of the LED chip is subjected to wavelength conversion with a fluorescent substance, and the light thus converted and light not subjected to wavelength conversion are mixed and emitted.
Specifically, there has been proposed, in order to emit white light, a light emitting unit provided with a wavelength converter including a fluorescent substance disposed on the surface of the LED chip. For example, in a light emitting unit in which a wavelength converter including a YAG fluorescent substance expressed by a composition formula of (Y, Gd)3(Al, Ga)5O12 is formed on a blue LED chip using an n-type GaN material, a blue light is emitted from the LED chip, and part of the blue light is changed to yellow light by the wavelength converter. Therefore, there has been proposed a light emitting unit in which the blue light and the yellow light are mixed to form white light (refer to Japanese Patent No. 2927279 and Japanese Unexamined Patent Publication No. 11-261114).
An example of the light emitting unit so constructed is shown in FIG. 4. In FIG. 4, a light emitting unit has, on a substrate 102 with an electrode 101 formed thereover, a light emitting diode 103 including a semiconductor material which emits light with a center wavelength of 470 nm, and a wavelength converter 104 disposed so as to cover the light emitting diode 103. The wavelength converter 104 contains a fluorescent substance 105. As required, the side surfaces of the light emitting diode 103 and the wavelength converter 104 may be provided with a reflector 106 reflecting light in order that the light escaping from the side surfaces can be collected to the front so as to enhance the intensity of an output light.
In this light emitting unit, when the light emitted from the light emitting diode 103 is irradiated to the fluorescent substance 105, the fluorescent substance 105 is excited to emit visible light, and this visible light is used as output. However, when the brightness of the light emitting diode 103 is changed, the ratio of quantity of light of blue to yellow is changed and hence the tone of white is changed, resulting in poor color rendering property.
In order to solve this problem, it has been proposed to emit white light by converting purple light to the wavelengths of red, green, and blue, respectively. That is, as shown in FIG. 5, a purple LED chip having a peak of 400 nm or less is used as a light emitting diode 103, and a structure that three fluorescent substances 115, 116, and 117 are mixed in polymer resin is employed as a wavelength converter 104 (refer to Japanese Unexamined Patent Publication No. 2002-314142). This enables the color rendering property to be improved.
Nevertheless, in the light emitting unit as described in the publication No. 2002-314142, the luminous efficiency of red component fluorescent substance (for example, Y2O3S:Eu, or the like) with respect to an ultraviolet region in the vicinity of 400 nm of excited light is considerably lower than the other fluorescent substances, failing to achieve white light offering a good luminous balance of red, green, and blue.
To overcome this, when the amount of blending of the red fluorescent substance having low luminous efficiency is increased, the red fluorescent substance reabsorbs the fluorescence emitted from the green and blue fluorescent substances. Hence, the amounts of emission of the green and blue fluorescent substances become lessened, failing to improve the luminous efficiency of the white light emitting unit. Although the luminous efficiency can also be improved by increasing the amount of blending of the green and blue fluorescent substances having high luminous efficiency, it is impossible to obtain white light offering a good luminous balance of red, green, and blue.
Consequently, there has been a demand for a photoluminescence fluorescent substance having high fluorescent quantum efficiency in a wide wavelength region from blue to red. As means for fulfilling this demand, for example, Japanese Unexamined Patent Publications No. 2004-71908 and W O 1999/050916(JP 2002-510866 A1) have proposed to form a photoluminescence fluorescent substance into nano-particles. Further, Bhargava et al reported in 1994 that, when the radii of particles are Bohr radius or less, the fluorescent quantum efficiency of a doped fluorescent substance is increased [refer to R. N. Bhargava et al, “Optical Properties of Manganese-Doped Nanocrystals of ZnS,” Phys. Rev. Lett. 72, 416 (1994)].
After this was reported, various considerations of the luminous characteristics of nano-particles have been made. As described in the above-mentioned publications No. 2004-71908 and W O 1999/050916, the representative example is to increase the luminous efficiency by utilizing quantum size effect to be developed by forming the photoluminescence fluorescent substance into nano-particles. Specifically, without changing any composition, the luminous efficiency can be increased by changing the particle diameter of the photoluminescence fluorescent substance from a several μm as has been conventional, to 0.1 nm to 100 nm.
However, the embodiment of WO1999/050916 discloses only one case where cadmium selenide (CdSe) is used in a core, as a photoluminescence fluorescent substance composition.
Although CdSe has a band gap of about 2 eV and can change the particle diameter to change the fluorescent wavelength, it has a strong toxicity that may raise questions of industry and environment. Also in the specifications of the above-mentioned publications No. 2004-71908 and WO 1999/050916, ZnS and the like are disclosed as an example of the fluorescent substance for which neither Cd nor Se is essential. However, this poses a problem that the fluorescent substance formed of such a composition cannot achieve such a luminous efficiency as high as CdSe.